std::memcmp

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Strings library

Null-terminated strings

Byte strings

Multibyte strings

Wide strings

Classes

basic_string

basic_string_view (C++17)

char_traits

 

Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C++11)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to numeric formats
atof atoiatolatoll (C++11) strtolstrtoll (C++11)	strtoulstrtoull (C++11) strtofstrtodstrtold (C++11)(C++11) strtoimaxstrtouimax (C++11)(C++11)
String manipulation
strcpy strncpy strxfrm	strcat strncat
String examination
strlen strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtok
Memory manipulation
memchr memcmp memset	memcpy memmove
Miscellaneous
strerror

 

Defined in header <cstring>
int memcmp( const void* lhs, const void* rhs, std::size_t count );

Reinterprets the objects pointed to by lhs and rhs as arrays of unsigned char and compares the first count characters of these arrays. The comparison is done lexicographically.

The sign of the result is the sign of the difference between the values of the first pair of bytes (both interpreted as unsigned char) that differ in the objects being compared.

Parameters

lhs, rhs	-	pointers to the memory buffers to compare
count	-	number of bytes to examine

Return value

Negative value if the first differing byte (reinterpreted as unsigned char) in lhs is less than the corresponding byte in rhs.

​0​ if all count bytes of lhs and rhs are equal.

Positive value if the first differing byte in lhs is greater than the corresponding byte in rhs.

Notes

This function reads object representations, not the object values, and is typically only meaningful for trivially-copyable objects with no padding. For example, memcmp() between two objects of type std::string or std::vector will not compare their contents, and memcmp() between two objects of type struct{char c; int n;} will compare the padding bytes whose values may differ when the values of c and n are the same.

Example

Run this code

#include <iostream>
#include <cstring>
 
void demo(const char* lhs, const char* rhs, std::size_t sz)
{
    std::cout << std::string(lhs, sz);
    int rc = std::memcmp(lhs, rhs, sz);
    if(rc == 0)
        std::cout << " compares equal to ";
    else if(rc < 0)
        std::cout << " precedes ";
    else if(rc > 0)
        std::cout << " follows ";
    std::cout << std::string(rhs, sz) << " in lexicographical order\n";
}
 
int main()
{
    char a1[] = {'a','b','c'};
    char a2[sizeof a1] = {'a','b','d'};
 
    demo(a1, a2, sizeof a1);
    demo(a2, a1, sizeof a1);
    demo(a1, a1, sizeof a1);
}

Output:

abc precedes abd in lexicographical order
abd follows abc in lexicographical order
abc compares equal to abc in lexicographical order

See also

strcmp	compares two strings (function)
strncmp	compares a certain number of characters from two strings (function)
C documentation for memcmp